1. Field of the Invention
This invention relates to a signal processing circuit of a solid-state image pickup device an image pickup section of which includes a plurality of solid state image sensors each having a discrete pixel array.
2. Related Art Statement
In a solid-state image pickup device the image pickup section of which includes solid-state image sensors each having a discrete pixel array formed by charge coupled devices (CCDs), aliasing components from the spatial sampling frequency fs are admixed into the image pickup output signals of the solid-state image sensors, as shown by hatched lines in FIG. 1. This is because the solid-state image sensors are of the sampling system.
The conventional practice to prevent the aliasing component from being produced into the base band of the image pickup output signal is to provide a double refraction type optical lowpass filter in the image pickup optical system to suppress the high frequency part of the baseband component of the image pickup signal to meet the Nyquist conditions of the sampling system by the solid state image sensors.
In the field of color video cameras, there are placed on the market a multi-plate type solid state image pickup device, such as a two-plate type solid state image pickup device in which three prime color images are picked up by a solid-state image sensor provided with color coding filters for red-color and blue-color pixels and a solid-state image sensor for green-color image pickup, or a three-plate type solid-state image pickup device for picking up of the three prime color images by separate color solid-state image sensors.
As a technique of improving the resolution in the multi-plate type solid-state image pickup device, there is known a so-called spatial pixel shifting method, according to which the cut-off frequency of the optical low-pass filter is selected to be fs and, as shown in FIG. 3, the solid-state image sensors for red color image pickup and blue color image pickup are placed with a shift equal to one half the spatial pixel sampling period with respect to the solid-state image sensor for green color image pickup.
By taking advantage of the spatial pixel shifting method, it becomes possible with the multi plate type analog output solid-state image pickup device to realize high resolution exceeding the limit imposed by the number of pixels of the solid state image sensor.
In service digital video tape recorders, employed in, for example, a broadcasting station, standardization of so-called D1/D2 formats are under way and digital interfaces for digital video related equipment conforming to these standards are required to be used in color television camera devices. According to the digital interface standards for the digital video related equipment, the sampling rate is set so as to be approximately equal to the sampling rate fs of the currently employed solid state image sensors.
In the solid state image pickup device the image pickup section of which includes solid state image sensors each having a discrete pixel array, such as the aforementioned CCD image sensors, a double refraction type optical low pass filter is provided in the image pickup optical system to prevent the occurence of the aliasing component into the baseband of the image pickup signal by the solid state image sensors. However, since the optical low pass filter cannot be made to have steep cut-off characteristics, the problem is presented that the modulation transfer function or MTF in the high frequency range is liable to be deteriorated in the high frequency range, as shown in FIG. 2.
Hence, in the multi-plate type solid state image pickup device for color image pickup, signal deterioration caused by the aliasing component or MTF occurs unavoidably if ordinary sampling is employed.
Meanwhile, when a digital interface for a digital video related equipment is loaded on a solid state imaging device, the image pickup section of which includes solid state image sensors each having a discrete pixel array, such as the CCD image sensors, the digital rate is too high with the sampling rate of the digital interface of 2fs, while the conformity to the digital interface standards cannot be achieved. On the other hand, if the rate of the digital processing system conforming to the D1/D2 format is adopted, the resolution becomes lower than that of the conventional solid state image pickup device for analog application when directly using the analog output.